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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Kees Cook <keescook@chromium.org>
Cc:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
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Date:   Tue, 10 Mar 2020 16:02:48 -0500
In-Reply-To: <202003101352.28BE3BEB4@keescook> (Kees Cook's message of "Tue,
        10 Mar 2020 13:55:54 -0700")
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Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
Sender: linux-kernel-owner@vger.kernel.org
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Kees Cook <keescook@chromium.org> writes:

> On Mon, Mar 09, 2020 at 02:02:37PM -0500, Eric W. Biederman wrote:
>>     exec: Add exec_update_mutex to replace cred_guard_mutex
>>     
>>     The cred_guard_mutex is problematic as it is held over possibly
>>     indefinite waits for userspace.  The possilbe indefinite waits for
>>     userspace that I have identified are: The cred_guard_mutex is held in
>>     PTRACE_EVENT_EXIT waiting for the tracer.  The cred_guard_mutex is
>>     held over "put_user(0, tsk->clear_child_tid)" in exit_mm().  The
>>     cred_guard_mutex is held over "get_user(futex_offset, ...")  in
>>     exit_robust_list.  The cred_guard_mutex held over copy_strings.
>
> I suspect you're not trying to make a comprehensive list here, but do
> you want to mention seccomp too (since it's yet another weird case).

I was calling out all of the places I have found so far where
cred_guard_mutex is held over waiting for userspace to maybe do
something.  Those places are what cause our deadlocks.

>> [...]
>>     Holding a mutex over any of those possibly indefinite waits for
>>     userspace does not appear necessary.  Add exec_update_mutex that will
>>     just cover updating the process during exec where the permissions and
>>     the objects pointed to by the task struct may be out of sync.
>
> Should the specific resources be pointed out here? creds, mm, ... ?
>
> But otherwise, yup, looks sane:

Probably not.  The design is if exec changes it we will hold the
cred_guard_mutex over it, so things are semi-atomic.

> Reviewed-by: Kees Cook <keescook@chromium.org>

Eric